Then, they don't get a license. In the scenario where the prospective licensee pays up front, then takes the test, and fails the test, the FCC would have to issue a refund if the prospective licensee didn't pass a test in a given timeframe.What happens when you do all the paperwork and testing, yet the applicant delays or never pays the FCC?
If the FCC worked it like they did back when they were charging for vanity callsigns, you had to pay at the time you made your application (within a certain number of days), and if your application was unsuccessful, you had to apply for a refund. That's a paperwork headache for a Federal agency that is already underfunded, understaffed, and continually struggles with a 1990's vintage computer database system.